In the past, the construction machines having a swing structure such as a hydraulic excavator were dominated by those equipped with an engine to drive a hydraulic pump delivering a hydraulic pressure for rotating a hydraulic motor that in turn drives the swing structure acting as an inertia field. In recent years, hybrid-type construction machines have been proposed which have the swing structure driven by both an electric motor and a hydraulic motor, the electric motor being powered by electrical energy from an electrical storage device, thereby helping to improve engine fuel efficiency, lower noise level, and cut down on the amount of exhaust fumes (see Patent Document 1). The construction machine described in Patent Document 1 includes not only an electric motor for swing purposes but also another electric motor for generating and supplying electrical energy to the electrical storage device when the level of stored electricity in the electrical storage device is lowered.
Patent Document 1 discloses a technique involving torque command means as control means for the hybrid-type construction machine, the torque command means commanding the torque to be in effect during drive control of the electric motor in connection with the pressure difference between the in-side and the out-side of the hydraulic motor for swing drive purposes. Also, Patent Document 1 discloses a technique whereby the ratio of the torque of the hydraulic motor to that of the electric motor during acceleration driving, and the ratio of the torque of the hydraulic motor to that of the electric motor during deceleration driving are determined using the pressure difference between the in-side and the out-side of the hydraulic motor as a parameter (e.g., paragraph 0060). Patent Document 1 explains that these techniques permit successive smooth drive control of the swing structure as an inertial field and enable the energy stemming from braking to be effectively charged into the electrical storage device as electrical energy (e.g., paragraphs 0033 and 0034).